Flare burners are well known in the art of petroleum processing and refining. See, for example, U.S. Pat. Nos. 4,052,142; 4,070,146; 4,643,669; 4,952,137; and 5,823,759. Typically, a flaring system is provided in a refinery or petrochemical plant to ensure the safe and efficient disposal of relieved gases or liquids, as may occur during normal plant operations or during emergency shutdown of such operations. The disposal fluids are collected in a flare header and routed to the burner. A flare burner is extremely important in the event of a plant emergency such as a fire or power failure, and a properly operating flare system is a critical component to prevent a plant disruption from turning into a disaster. A flare burner must always be available for flaring whenever a plant disruption occurs. A flare burner is expected to be operable twenty-four hours a day and typically must be in service for several years without a need to shut it down. A flaring system must reduce ground level concentrations of hazardous materials, provide safe disposal of flammable materials, and reduce volatile organic compound (VOC) and hydrocarbon emissions.
Flare burners typically are oriented to fire upward. The discharge point is in an elevated position relative to the surrounding grade and/or nearby equipment. Some flare burners, known in the art as enclosed flares, are constructed to conceal the flame from direct view, which can reduce noise and minimize heat and sound radiation. Multiple stages within the enclosed flares are sometimes used.
A single-point flare burner is an open pipe tip with a single gas exit point, and may be of the smokeless or non-smokeless design. Smokeless flares eliminate any noticeable smoke over a specified range of gas flows. Smokeless combustion is achieved by utilizing auxiliary air, steam, or other means to create turbulence and entrain air within the flared gas stream to improve combustion. The flame produced by a gas-assisted pipe flare burner is a function of the relief gas characteristics, the gas exit velocity, and the gas injection design. For economic reasons, steam is a currently-preferred gas for use in gas-assisted pipe flare burners. Compressed air or other high-pressure gases, including light molecular weight hydrocarbon vapors, can be used, but steam has been found to be the most cost-effective medium. Note, however, that as used herein, “steam” should be taken to mean any gas used for educting air into a flare burner.
Steam injection functions to produce smokeless combustion by educting combustion air into the combustion zone at the exit of the gas supply pipe, thus increasing momentum and turbulence in the flare flame. The combination of educted combustion air, momentum, and turbulence can produce short, intense flames.
Steam often is injected into the gas discharge at the top of a flare burner as well (see, for example, U.S. Pat. No. 4,643,669). Typically, a steam ring having a plurality of injection nozzles or slots is disposed on the outside of a burner shell, the nozzles angled inwards to draft air into the combustion zone. The steam and air act to dilute the hydrocarbon fuel content, which also reduces smoking tendency. The steam vapor can also participate in the combustion kinetics, assisting in the conversion of carbon to carbon monoxide.
In prior art flare burners, upper steam rings can be subject to flame impingement due to wind action. However, an upper steam ring also may function as a windshield to reduce adverse wind effects on the flare flame and can also help to prevent undesirable flame attachment to the outer surface of a flare burner shell. Both of these problems are well known in the art. A flare burner that employs both internal steam/air tubes and an upper steam ring may have more than twice the maximum smokeless burning capacity of an upper steam ring flare. The steam/air discharge from the internal tubes can also be at a high velocity, in some burners approaching Mach 1, adding to the momentum of the flare discharge and inspirating additional combustion air while stiffening and shortening the flame.
Back burning is a potential hazard with prior art steam/air tube flare burners. Care must be taken to avoid back flow of combustible mixtures into the internal tubes and feed pipe. The most common cause of back flow in the tubes is improper flare operation. If the upper steam ring is pressurized prior to engaging the steam supply to the steam/air tubes, the upper steam can cap the top of the flare discharge and force flow backward out of the steam eductor tubes.
Prior art gas-assisted flare burners are subject to numerous well-known shortcomings which can affect performance and working life of a flare burner. For example, the upper steam ring and steam supply piping typically are welded via brackets to the outside of the burner shell, which in use can result in severe thermal distortion and fracture of the ring and/or shell. Typically, the shell is not especially reinforced at the upper end and thus is vulnerable to such distortion. Temperature differentials of many hundred degrees may be produced over a distance of only a few inches. An upper steam distribution ring and steam injectors on the outside of the shell can also provide anchor points for uncontrolled fires on the outside of the burner shell, and may actually incite such fires through turbulence of air and gas around the steam ring.
Further, prior art steam/air tubes are entered into the burner diagonally through openings in the shell sidewall and include welded elbow turns of 30-60° within the shell, which elbows reduce air eduction rates significantly and are also known to fracture from thermal and vibratory stress. As noted above, gas flow rates through the tubes can approach Mach 1, creating great stress on the tubes and especially on the elbows therein. Also, this arrangement requires a large and cumbersome steam injection assembly surrounding the inlet ends of the tubes where they protrude through the sidewall of the shell. Also, the number of tubes within the burner shell is limited by this arrangement, and relatively few tubes are provided toward the center of the burner. Thus the lateral distribution of educted air within the flare is non-uniform.
Further, a pilot ignition system typically is attached by welded brackets onto the outer surface of the shell near the upper edge, along with the steam ring, which can also result in thermal distortions and uncontrolled burning outside the shell.
Further, under conditions of low gas flow, the velocity of the air gas mixture at the flare outlet can be insufficient to prevent turbulent re-entry of the mixture and the ignition front into the burner, creating a potentially explosive situation.
What is needed is a flare burner having increased resistance to fires outside the shell, less restrictive steam/air tubes, a robust pilot ignition system at the exit to the burner, and an improved gas velocity seal. What is further needed is a flare burner having all steam/air tubes and pilot ignition sub-assemblies disposed within the shell, having all straight steam/air tubes with no bends, and having no hardware welded or attached to the outside of the shell near the open end thereof.
It is a principal object of the present invention to increase the working lifetime of a flare burner.
It is a further object of the present invention to increase the reliability of a flare burner.
It is a still further object of the present invention to reduce the cost and complexity of a flare burner.
It is a still further object of the present invention to improve the mechanical durability of a flare burner.